In many communication systems, e.g., telephone systems, it is common to connect two-wire local lines to the four-wire trunk lines via a hybrid or other isolation network. The four-wire system provides separate one-way paths for transmission and reception of signal energy while the two-wire local circuit provides a single two-way path between the subscriber and the terminal at which the two- and four-wire systems are connected. When the signal energy on one path of the four-wire circuit is coupled into the two-wire local circuit through the hybrid, part of the signal is returned over the other path of the four-wire line back to the terminal where the signal originated due to impedance mismatches at the hybrid. This returning signal is known as echo.
When the transmission path length is relatively short, the time delay between the transmission of speech from a subscriber set to the return of the echo at that set is very short, and the echo will present little or no problem. However, in longer transmission systems, such as long line systems or systems employing geosynchronous satellites as repeater stations, the time delay can be in excess of a half second. An echo which occurs a half second after all transmitted speech would be extremely annoying to the subscriber and, therefore, it is necessary to provide some type of echo attenuation.
Echo suppressors have been used as a partial solution to this problem. See, for example, U.S. Pat. Nos. 3,896,273 and 3,206,559. Conventionally, a half echo suppressor is located near each hybrid terminal on the four-wire side of the system. When a far-end talker begins to speak, his signal is detected by a voice detector in the echo suppressor of the near-end talker. The voice detector then causes a switch to open the echo return transmission path to prevent the echo from returning to the far-end talker. A break-in feature is typically provided so that if the near-end talker begins to speak while the far-end talker is still speaking, the transmission path will be enabled. This is accomplished by providing a break-in switch in parallel with the echo suppression switch to bypass the open echo suppression switch when the level of signal activity at the near-end transmission path is greater than that on the near-end received path.
For purposes of explanation, a simplified functional block diagram of an echo suppressor used in long distance telephone communications is shown in FIG. 1. For a detailed explanation of its operation, reference is made to CCITT Recommendation G.161 (Orange Book), Sixth Plenary Assembly, 1976, Volume 5. In brief, its operation is as follows.
Received speech present on four-wire transmission path 1 is recognized by a speech detector 2, which causes normally closed switch 5 to open. Due to impedance mismatches in the hybrid 8, a portion of the speech signal on receive line 1 will pass through the hybrid 8 to the transmit line 12. The opening of switch 5 will prevent this echo signal from returning to the far-end speaker. When the receive speech on line 1 terminates, switch control 4 continues to keep switch 5 open for a hangover time interval set by delay 9.
The break-in mode is activated when the near-end talker 10 wishes to interrupt the far-end talker who is generating the received speech on line 1. In this case, the speech detector 11 compares the levels of the transmit and receive speech, and when the signal level on the transmit line 12 is equal to or greater than the receive speech on line 1, speech detector 11 will activate a switch control 13. The switch control causes a normally open switch 6 to close, thus bypassing the open switch 5 and permitting the near-talker speech to be transmitted to the far end. Simultaneously, a normally closed switch 7 will open, thus causing a loss 14 to be inserted into the receive speech path. In some echo suppressors, switch 7 is opened when transmit speech is present on line 12, regardless of whether or not receive speech is present on line. When the double-talk condition stops, switches 6 and 7 are maintained in their open and closed positions for hangover times determined by delays 15 and 16, respectively. During double-talk, the level of speech heard by the near talker 10 is reduced by the loss 14, but some echo from the received speech line 1 is still transmitted along with the near-talker speech on the transmit line 12.
Although echo suppression systems of the above-described type have been found suitable for most telephone system applications, there are certain serious disadvantages which arise from the use of such conventional systems in a speaker phone system. First, there is a tendency for the suppressor to break in on its own echo, i.e., the echo level on the transmit line 12 may be sufficient to trigger the break-in mode. This will falsely activate the double-talk circuitry and permit a burst of echo to occur. Such an echo burst tends to be more noticeable than in a conventional telephone call since there may be a number of "third-party listeners" participating in the teleconference who are not involved in the conversation and are, therefore, more critical of what they hear. A second problem is that a conventional echo suppressor tends to respond quickly to noise bursts due to open microphone pickup of noises such as finger tapping, microphone handling, clothes movement, etc. If this noise is considered by the suppressor to be speech, the double-talk mode of operation will be triggered. This will permit an echo to return to the far-end talker and the opening of switch 7 will simultaneously reduce the speech level for the listeners. An additional problem is that, in those echo suppressors in which the switch 7 is opened during speech transmission, regardless of whether or not a speech signal is being received, anytime a person in the room is speaking, the background noise emanating from the speaker phone is reduced. This modulation of background noise is annoying to listeners in the same room as the speaker.
In teleconferencing via a four-wire connection, the echo is caused by the acoustical feedback from the loud-speaker to the microphone rather than by a hybrid. The sound propagation velocity in air is approximately 1 ft/ms. The direct sound end delay time in milliseconds corresponds to the loud-speaker(s)-to-microphone(s) distance in feet and normally exceeds the telephone echo suppressor's typical 3 ms suppression operate time. The reverberated sound delay depends on the sound absorption of the reflecting surfaces in the room and the room volume, and may exceed the typical 50 ms suppression hangover time of the telephony echo suppressor.
A different type of echo suppression which is commonly used in speaker phone systems is the use of a variable loss device in either one of the transmitting and receiving channels during talking. Such a system is described in U.S. Pat. No. 3,952,166. That system includes a variable loss element which is varied in dependence on the loud-speaker-to-microphone distance in order to compensate properly for the acoustic coupling between the speaker and microphone. An undesirable feature of such a system is that the variable loss device will cause an annoying sound level modulation to the listener. Further, such a system has a tendency to sound hollow, depending on the volume of the talker and the size of the room in which the loud-speaker is located. In such systems, longer propagation delays require longer losses, and in some cases, the amount of loss required may increase to a point where it interferes with the transfer of information between parties.
Echo, hollow sounding speech and speech level modulation significantly detract from the quality of the teleconference. Thus, there is a need for an improved echo suppression system which will substantially alleviate these problems. The present invention is specifically directed to a system which will alleviate the above-described problems in a video teleconference system, which is essentially a speaker phone system accompanied by a visual image.